Carbon dioxide is a worldwide-recognized main greenhouse gas which causes the global warming. With the rapid development of the economy, the carbon dioxide emission of China is more than 6 billion tons annually. It has surpassed the United States, making China the country with the most carbon dioxide emission in the world. Thus, carbon dioxide emission reduction has become an important issue that urgently needs to be addressed in China.
Among carbon dioxide capture methods, the most popular one is the chemical looping method, where carbon dioxide is absorbed through the carbonation reaction of calcium oxide and carbon dioxide, then formed calcium carbonate is decomposed to re-obtain calcium oxide at a high temperature. Calcium oxide has a high sorption capacity of 0.786 g CO2/g CaO at a high temperature of 600° C. and a fast adsorption rate, and even can achieve the purpose of continuous removal of carbon dioxide in a flue gas by employing a circulating fluidized bed system.
It has now been found through research that the cost of carbon dioxide capture is mainly related to the cost of calcium oxide adsorbent and the stable sorption properties of the adsorbent (MacKenzie A, Granatstein D L, Energy Fuels. 2007, 21(2): 920-926). As the high-temperature carbon dioxide adsorbent calcium oxide derived from high-temperature calcination of natural minerals such as hydrotalcite, dolomite and montmorillonite has the advantages of abundant sources and low cost. But its disadvantage is the low adsorption ratio as well as sorption capacity decaying rapidly in the process of carbonation and regeneration cycles, which may lead to the high cost of carbon dioxide adsorbent. The synthetic CaO-based carbon dioxide adsorbent improved from the perspective of adsorption performance has a higher sorption capacity than natural minerals, but a high cost of raw materials and preparation process is the key problem restricting the industrial application of CaO-based CO2 adsorbents.
Therefore, selecting a kind of a low-cost material containing calcium oxide to be the precursor to prepare an inexpensive calcium oxide adsorbent with excellent adsorption performance and a low cost is an urgent problem needs to be solved.
Phosphogypsum is a solid industrial waste generated from wet phosphoric acid production process, and making 1 ton of phosphoric acid produces 4.5-5 tons of phosphogypsum. With the rapid development of phosphate fertilizer and efficient phosphate industry, the emission of phosphogypsum waste is increasing. In the world, annual phosphogypsum emission amount is about 280 million tons; and China's annual phosphogypsum emission has been more than 50 million tons, accounting for 70% of the industrial gypsum by-products. If there is not a reasonable treatment and utilization, the abundant stockpile of phosphogypsum will restrict the development of the industry as a whole because of the existence of phosphorus pentoxide, fluorine and free acid and other harmful substances in phosphogypsum. Currently, the accumulated stockpile of phosphogypsum in China has more than 250 million tons. However, the effective utilization ratio is less than 20%, even though worldwide average utilization ratio of phosphogypsum is only 4.5%, generally used for low-level applications such as building materials, cement, soil improvement, or directly as a roadbed.
There is generally more than 90% calcium sulfate dehydrate in phosphogypsum as well as undecomposed phosphate rock, residual phosphoric acid, calcium fluoride, the oxide of silicon, iron, aluminum, magnesium, etc. acid insoluble, organic matter and so on. To some extent, the presence of these impurities has an impact on the utilization of phosphogypsum as a chemical resource. So far, the chemical treatment methods of phosphogypsum are as follows: phosphogypsum reduced by carbon at high temperatures to prepare sulfuric acid combined production cement; joint production of hydrogen sulfide and calcium carbonate; joint production of ammonium sulfate and calcium carbonate.
China Patent Publication No. CN101492178A discloses a process for joint production of ammonium sulfate and calcium carbonate from phosphogypsum. In this process, firstly, after size mixing, rinsing, sedimentation and sieving, phosphogypsum's whiteness reaches to 75-85%. Then an additive and ammonium carbonate were added into the phosphogypsum slurry, and the reaction temperature was kept at 30-60° C. for 0.5-2 hours. After that, the temperature of the slurry was reduced to room temperature, the slurry was filtered and water was added to filter cake to get calcium carbonate slurry. Then a surface modifier was added, and after a certain reaction time, the product was filtered and dried to obtain calcium carbonate of a purity greater than 93% with an average particle diameter of 1 to 5 microns.
Zhang Wan-fu's study named “Industrialization analysis of ammonia sulphate from phosphogypsum” (Chemical Engineering, 2009, 37 (011): 75-78) reports the optimal reaction conditions of the reaction of phosphogypsum and ammonium carbonate to prepare calcium carbonate. The conditions are as follows: the reaction temperature is 60° C., the reaction time is 1 hour, the molar ratio of ingredients (ammonium carbonate to phosphogypsum) is 1:1, the mass ratio of liquid to solid (water to phosphogypsum) is 5:1, the amount of ammonia (molar ratio of ammonia to phosphogypsum) is 0.8:1, and stirring speed is 300 r/min, thus the obtained calcium carbonate is mainly hexagonal system calcite type.
China Patent Publication No. CN101993105A discloses a process for preparation of precipitated calcium carbonate from phosphogypsum with phase transfer agents such as ammonium sulfate, ammonium salt or sodium salt of an organic acid. In this process, phosphogypsum is transformed into a soluble calcium ion solution by phase-transfer reaction, and then a mixture of ammonium bicarbonate and ammonia or carbonation agent of solid ammonium carbonate is added to the solution. The precipitate is separated to obtain the calcium carbonate with a purity of more than 97% and particle size of about 1 micron.
In the above literatures, the calcium carbonate prepared from phosphogypsum acts as inorganic filler, which requires high purity, thus leading to complicated process and high cost.
China Patent Publication No. CN101337685A is disclosed a process for preparation of calcium carbonate using solid residues obtained from phosphogypsum and carbon pyrolysis to absorb carbon dioxide produced in the phosphate industry at 25-80° C. The calcium carbonate mass content is 75-85%. It is also used as a filler after treatment.
Currently, there are few studies about phosphogypsum treated to be a carbon dioxide adsorbent. The literature (C Cárdenas-Escudero, V Morales-Flórez, etc. Journal of Hazardous Materials, 2011, 196 (0): 431-435) reports that sodium hydroxide solution was added into a phosphogypsum slurry, and then the slurry is filtered to obtain the calcium hydroxide filter cake. The filter cake is dispersed in water, and then carbon dioxide is bubbled in at lower than 80° C. or at room temperature to complete wet carbon dioxide capture. The feature of this process is that it uses phosphogypsum to prepare calcium hydroxide and then the calcium hydroxide reacts with carbon dioxide to capture carbon dioxide. The carbonation reaction is occurred at room temperature and the carbon dioxide passed into is pure.
Since “eleventh five-year plan” a large number of desulphurization facilities in thermal power plants have been built up and put into production in China. More than 90% of the desulphurization facilities in China employ the limestone-gypsum desulfurization process. However it results the production of a large number of desulfurization by-products—FGD gypsum. In 2010, annual output of FGD gypsum in China is more than 20 million tons. At present, the utilization ratio of the FGD gypsum in China is only 30%. A large number of FGD gypsum is stacked or filled in the sea. It not only occupies a lot of land but also cause secondary pollution because of the sulfur off back to the ground.
Fluorine gypsum is the by-product of preparation of hydrofluoric acid using fluorite and concentrated sulfuric acid in the fluoride salt factory. Due to the residues of a certain amount of sulfuric acid or hydrogen fluoride in fluorine gypsum, directly stockpiling of fluorine gypsum not only occupies the land, but also pollutes the soil and groundwater environment, causing serious environmental pollution in the stacking process.